The present invention relates to the technical field of actuating fuel injectors. In particular, the present invention relates to a method for determining an injection quantity of a fuel injector, having a solenoid drive, for an internal combustion engine of a motor vehicle. The present invention also concerns a method for actuating a fuel injector having a solenoid drive, wherein the actuation is based on an injection quantity determined according to the invention. The present invention furthermore relates to an engine controller and to a computer program which are designed to carry out the method according to the invention.
In order to inject fuel into a combustion chamber, such as a cylinder for example, a fuel injector such as, for example, a solenoid valve or a solenoid injector may be used. A solenoid injector (also called a coil injector) of this kind has a coil which generates a magnetic field when current flows through the coil, as a result of which a magnetic force is exerted on an armature so that the armature moves in order to cause opening or closing of a nozzle needle or of a closure element for opening or closing the solenoid valve. If the solenoid valve or the solenoid injector has a so-called idle stroke between the armature and the nozzle needle, or between the armature and the closure element, a movement of the armature does not also lead to a movement of the closure element or the nozzle needle immediately, but rather only after a movement of the armature by the magnitude of the idle stroke has been completed.
When a voltage is applied to the coil of the solenoid valve, electromagnetic forces move the armature in the direction of a pole piece or pole shoe. After overcoming the idle stroke, the nozzle needle or the closure element likewise move owing to mechanical coupling (e.g. mechanical contact) and, with a corresponding shift, opens injection holes for the supply of fuel into the combustion chamber. If current further flows through the coil, the armature and nozzle needle or closure element continue to move until the armature reaches or stops against the pole piece. The distance between the stop of the armature on a carrier of the closure element or the nozzle needle and the stop of the armature on the pole piece is also called the needle stroke or working stroke. In order to close the fuel injector, the exciter voltage which is applied to the coil is switched off and the coil is short-circuited, so that the magnetic force is dissipated. The coil short-circuit causes a reversal of polarity of the voltage owing to the dissipation of the magnetic field which is stored in the coil. The level of the voltage is limited by a diode. The nozzle needle or closure element, including the armature, is moved to the closing position owing to a return force which is provided, for example, by a spring. The idle stroke and the needle stroke are run in reverse order here.
For short injection times, the closing process begins even before the armature has stopped on the pole piece, so the needle movement thus describes a ballistic trajectory.
The time of starting the needle movement on opening of the fuel injector (also known as OPP1) corresponds to the start of the injection, and the time of ending the needle movement on closing of the fuel injector (also known as OPP4) corresponds to the end of the injection. These two times therefore determine the hydraulic duration of the injection. Consequently, for identical electrical actuation, injector-specific temporal variations for the start of the needle movement (opening) and the end of the needle movement (closing) can lead to different injection quantities.
According to the prior art, the injection quantity is frequently estimated by multiplying the hydraulic duration by an assumed constant through flow rate. In the event of short injection times, for example in conjunction with multiple injections, in particular in cases in which the needle movement describes a ballistic trajectory, such estimations cannot ensure the necessary precision to be able to set a uniform injection by a plurality of fuel injectors.